A nonreactive pan is one that is made of or lined with a material that will not react with acidic ingredients such as tomatoes, citrus juice or vinegar. When using a pan that is not nonreactive it will result in an “off” flavor and an unappealing, but harmless, darker coloring of of the food. This is why a nonreactive pan is essential when cooking certain foods.
The most common nonreactive pans are made of anodized stainless steel but they can also be made from enamel or glass. On the flip side, reactive materials include nonanodized stainless steel, unlined copper or cast iron. Cast iron is the least problematic of the three as it can be used to prepare acidic foods only if the food is not left in the pan for an extended period of time.